Colonisation and adhesion of pathogens to solid surfaces is an ever present problem in agriculture, the food industry, the medical field and various other industries, as this can lead to spoilage of products, fruit and vegetables, chronic infections and/or the formation of antibiotic resistant biofilms.
In agriculture, post-harvest losses are in the region of 24% in the USA (U.S. Dept. of Agriculture, 1965, U.S. Dept. Agr. Hdbk., 291) and about 50% in underdeveloped tropical countries (Courtsey and Booth, 1972, Rev. Plant. Pathol., 51:751-765). Post-harvest spoilage can be prevented by the use of chemical sprays, dips or washes; fumigation; or the use of treated wrappers, box liners or shredded paper. Chemical treatments generally make use of borax, sodium ortho-phenylphenate, chlorine, antibiotics, diphenylamine or ethoxyquin. Sulfur dioxide, nitrogen trichloride, ammonia or ammonium compounds or carbon dioxide are usually used for fumigation. Packaging material is treated with biphenyl, orthophenylphenol, iodine, copper sulphate, mineral oil and diphenylamine. Biphenyl is used in wrappers or box liners, often in conjunction with other treatments such as borax (Godfrey and Ryall, 1948, Texas Agr. Expt Sta. Bul. 701-724; Harvey, 1952, Phytopath., 42: 514). This lowers the occurrence of blue and green molds on citrus and stem-end rots (Harvey and Sinclair, 1953, Rept. Tech. Com. Citrus Assoc. Univ. of Calif., 1-27), but rather than killing fungal infections, the vegetative growth and spore formation of citrus pathogens are merely inhibited. Biphenyl has also been found to stimulate the growth of some vegetable and fruit pathogens (Heiberg and Ramsey, 1946, Pytopath, 36:887-891). Orthophenylphenol-impregnated wrappers have been shown to be effective against some citrus fungi and to lower the infection of tomatoes, grapes and apples (Plank, Rattrey and van Wyk, 1940, Jour. Pomol. And Hort. Sci., 18:135-144). However, injury and scalding of the fruit has been observed after exposure to orthophenylphenol-impregnated wrappers. Iodine-impregnated wrappers have also been shown to have activity against blue mold, without damage to citrus, but iodine's volatile nature causes the inhibitory effect to wear off quickly (Smith, 1962, Botanical Review. 23(3):411-445). Other treatments are also known to prevent fungal infections, but with a range of drawbacks, of which damage to the fruit and vegetables is most prominent. A safe and effective antimicrobial treatment that does not damage fruits and vegetables would therefore be of great use in the agricultural industry.
In the medical field and related industries, the treatment of biofilms is primarily directed at the removal of mature biofilms. Resistance to currently available treatments has been observed and can be due to the exopolysaccharide matrix preventing the penetration of treatments into the entire biofilm, or differences in metabolism between layers of the biofilm and mixed organism biofilms, where not all the organisms are affected by the treatment. It has been found that the most effective way to remove biofilms consists of a combined effort including a treatment and physical removal of the biofilm, e.g. scrubbing or high pressure spraying. This, however, is not possible in all areas where biofilms could form.
Urinary catheters, which are typically made of silicone or latex, are an example in the medical field where biofilm formation is common. A study of catheter biofilms (Stickeler, 1996, Biofouling, 94:293-305) found that in instances where catheters were inserted for longer than 28 days, most patients developed an infection due to a biofilm found in the catheter itself. Commonly found organisms in catheter biofilms are Staphylococcus epidermis, Enterococcus faecalis, Escherichia coli, Proteus marabilis, Pseudomonas aeruginosa and Klebsiella pneumonia. It has been found that the hydrophobicity of both the organisms and catheter material determines the type of organisms found within the biofilm (Brisset, Vernet-Garnier, Carquin, Burde, Flament and Choisy, 1996, Pathol. Bio., 44:397-404). No specific catheter material has been found to possess the means to prevent colonisation of organisms (Tunney, Jones and Gorman, 1999, Doyle RJ (Eds), Methods in Enzymology. San Diego: Academic Press. 558-566). Control measures such as antimicrobial agents in collection bags, antimicrobial ointments and lubricants, the use of antibiotics and bladder installation and irrigation have been tested, but none of these showed significant results (Kaye and Hessen, 1994, IN: Bisno and Waldovogel (Eds), Infection associated with indwelling medical devices. Washington. American Society for Microbiology, 291-307). Only silver-impregnated catheter tubes, which delayed the attachment of organisms for 4 days, showed an improvement.
Wound dressings are another example within the medical field where surface contamination is problematic, often leading to infection of the wounds which the dressings are supposed to protect. Impregnated wound dressings with bactericidal, virocidal or fungicidal activity have been developed for use on wounds that are already infected, i.e. to not only aid the healing of the wound, but to also fight the infection that is preventing the healing of the wound. For example, Betadine™ wound dressing is impregnated with 10% povidone-iodine solution, has bactericidal and virucidal activity and is marketed for use on contaminated or superficially infected wounds (Herruzo-Cabiera, Vizcaino-Alcaide, Mayer, Rey-Calero, 1992, Burns, 18:35-37). A drawback of this dressing, however, is that it becomes stiff as it dries, which can cause discomfort to the patient and can also disrupt the wound. Silver impregnated dressings are also used to fight infected wounds, such as Acticoat™ (Westaim Biomedical Inc., Fort. Saskatchewan, Alberta, Canada) and SilverIon® (Argentum Medical, L.L.C., Lakermont, Ga.). Both of these dressings use nanocrystalline silver to release silver to the wound area in a controlled and prolonged manner, resulting in a lower frequency of changing of the dressing, lower risk of further infection, lower cost of treatment and preventing continuous patient discomfort and tissue damage. However, few studies have been conducted on the effect of silver on the wound bed, how it is metabolised or how it effects the overall healing process of the wound.
In the food industry, Listeria monocytogenes is commonly associated with biofouling and is found in meat and dairy processing plants. Although the sheer force used to clean pipes within the processing plants should be enough to remove exposed biofilms, it is the hard-to-reach places (such as cracks within equipment caused by age, gaskets, valves and joints) that are more likely to develop biofilms and these are difficult to remove. Furthermore, environmental surfaces (floors, walls and the like) have been found to be susceptible to extensive biofilm formation and can lead to the reintroduction of Listeria in a cleaned processing plant. In conjunction, resistance of Listeria to sanitizing agents used within the food processing environment has been observed. This is of great concern, since Listeria is responsible for 28% of deaths caused by the intake of contaminated food in the USA (Mead, Slutsker, Dietz, McCraig, Bresee, Shapiro, Griffin, Tauxe, 1999, Emerg. Infect. Dis. 5: 607-625).
One of the biggest problems with biofouling is that once an organism has adhered and colonised to a surface, it can form resistant biofilms that are difficult to remove completely, leaving a constant source for re-infection or chronic biofouling.
There is thus a need for new ways of preventing or treating microbial infections and biofilm production on surfaces, especially in the agricultural, food and medical industries or, in other industries that encounter instances of infections and/or biofouling.